As the bacteria work in the anaerobic outer ring they remove oxygen from the sewage. They are so hungry for oxygen they then remove the oxygen from the nitrates and nitrites leaving nitrogen.
The partly fixed sewage then flows to clarifier, which is the center circular portion of each plant. There sweep arms continuously stir up the solids and then pull them out. Some of the solids are “wasted” and sent to a 500k holding tank which is aerated and where aerobic bacterial digestion takes place. This further decomposes the sewage.
From the holding tank the sewage is removed in summer to be spread in drying fields which are the flat areas west of the two plants. Once it ís dried it’s trucked to landfill sites to become daily cover. Water entering the clarifier is at about 120 TSS (total suspended solids). Coming out of the clarifier, the concentration is at about 20 TSS (and can be as low as 10 TSS in summer).
That which is not solid is sent to the filtering building as second day effluent where remaining solids are coagulated using polymers. This makes the solids a little larger in size so they will be trapped by sand and charcoal filters. At this point the TSS level is less than 5. The plant is approved to allow a level of 20 TSS effluent level.
Chlorine is added to the resulting water and balanced with sulfur dioxide. Just enough chlorine is added to kill the remaining bacteria and then enough sulfur dioxide is added to remove the chlorine. Chlorine must be removed so that when the water enters the river it will not kill the fish.
Newer plants use UV disinfection to kill the bacteria. This prevents any chlorine from leaving the plants but also prevents trihalomethane from entering the river which is a by-product of the chlorine disinfection and can cause health problems. The State is mandating this change to UV disinfection but since the DSPUD permit runs until June 2007 (five year cycle from 2002) this has not yet been mandated for DSPUD.
As a percent of stream flow, the DSPUD plant can be as much as 60% in the fall/winter. From July to October no water is allowed to enter the river and so, when possible, it is sprayed on the lower portion of the Soda Springs ski hill. This is called “land application.” The water then evaporates. This is also a balancing act since the water cannot be allowed to enter the Yuba River source as it leaves Van Norden further downhill. Rain may cause cessation of spraying. Up to 500k gallon/day may be sprayed on the hill.
Since the effluent flows to the Yuba River at least some of the year there need to be safety considerations so that there wonít be too much chlorine or turbidity. Too much chlorine can kill fish and turbidity (large particles which may harbor bacteria) may be unsafe. If the balance is not correct, automatic valves shift the sewage flow to a 1.5M gallon emergency storage tank at the east end of the site.
Throughout the sewage treatment process constant monitoring takes place to be sure there are the correct balances and levels of, for example, BOD’s vs. bacteria, levels of conditions that may indicate e.Coli, chlorine and turbidity, alkalinity, suspended solids, oxygen levels, etc. Some of the monitoring is done electronically but much is also done manually and some reduntantly.
Go on the to page three of the field trip